Table of Contents
Examples of Cryogenic Applications
Thermal Transfer Performance
Vapor Pressure and Vacuum Applications
Cryogenics normally refers to operation at temperatures below 123 K, or -150 °C and in a number of branches of science and medicine it is essential to execute experiments and run equipment at cryogenic temperatures, down to as low as a few degrees Kelvin. At these temperatures, specialist materials are required in order to provide sealing and thermal conductance, as standard heat sink compounds and sealing formulations may craze or crack, which in turn reduces thermal conductivity and reduces sealing efficiency.
There are several types of grease on the market presently available, such as PFPE based products that quote operating temperatures much below 0 °C, however almost all these products have a quoted operating temperature range limit of -80 °C. Typical heatsink compounds are only quoted as working down to -50 °C. The one exception is hydrocarbon based grease, with proprietary additives which can work all the way down to -269 °C, the temperature at which liquid helium boils.i
This article discusses the demands of cryogenic applications and research on the behavior of hydrocarbon based grease for thermal contact and sealing.
Examples of Cryogenic Applications
There are a number of scientific fields and industries where cryogenic temperatures are utilized, two examples are superconductors and research.
In university research fields there are several examples of where samples need to be cooled to very low temperatures. In these experiments, it is essential to mount a sample onto a cryostat cold finger, in order to attain extremely low temperatures and then once the experiment is complete it is necessary the sample must be removed again. In this case, the compound used for mounting has to be pliable at room temperature, but harden at lower temperatures. It is also extremely useful if the compound can be effortlessly removed and cleaned from the sample after the experiment is complete.
Another key application for cryogenics is within superconductors, where extremely low temperatures are needed to produce the phenomenon of very low electrical resistance. The typical temperature for low temperature superconductors is <30 K, while high temperature superconductors operate at temperatures up to 138 K. Although considered high temperature, this is still well into the cryogenic range and needs the use of materials capable of withstanding very low temperatures without losing any of their properties.
Superconductors are considered to be key to the production of extremely high strength magnets, employed in devices such as Magnetic Resonance Imaging (MRI) scanners and maglev transport systems. More niche applications include particle accelerators and fusion reactors where very high power magnet fields are needed to provide the necessary containment and acceleration forces. Within these systems, there is a need for thermal contact between the superconducting materials and cooling equipment and also for accurate temperature measurement.
For temperature sensors it is vital that good thermal contact is maintained to provide accurate measurements. Thermowells are usually used where it is necessary to directly measure the temperature of helium or liquid nitrogen. The thermowell acts to guard the delicate temperature sensors from damage and also allows the placement of the temperature probe into the area of interest, for instance into the middle of a pipeline. One challenge to using a thermowell is that good thermal contact will have to be made between the temperature sensor and the outer wall.
Hydrocarbon grease is frequently used to fill thermo-wells in cryogenic equipment, in order to provide this necessary contact between the medium to be measured and the sensor. In this type of scenario, the grease is perfect as it is easy to apply to the thermo-couple, or insert into the well.
The fact that the grease will soften at room temperature means that the sensor head can be effortlessly removed from the thermowell for servicing or replacement if needed, something that is not possible with an epoxy resin, or similar permanently set thermal interface material. Once the probe is replaced and then cooled back down the grease hardens once more, providing exceptional thermal contact to the outer surface.
Figure 1. Thermowell application for grease.
One area where greases fail to provide a solution is in cryo-lubrication. Regardless of the base oil used greases will be a solid below around 170 K, thus unsuitable for providing lubrication while in operation. Where grease can be used is as an assembly lubricant, where the compound aids in the construction of a cryogenic system, such as allowing the insertion of a shaft through an o-ring, without damaging the elastomer. In this case, grease capable of withstanding thermal cycling is again useful as it permits the removal and replacement of the shaft for maintenance purposes. However, it should be noted that dynamic systems for operating at extremely low temperatures will need the use of solid lubricant coatings.
Thermal Transfer Performance
Since thermal transfer at cryogenic temperatures is a vital area where specially formulated grease can be used it is useful to look at the comparative performance in a more detailed manner. One of the vital problems with cryogenics is providing good heat transfer between the experimental object and the cooling equipment. For example, consider the mounting of a semiconductor chip on to a cryostat cold finger assembly. There is a need to provide excellent thermal contact between the two surfaces in order for the chip to be effectively cooled.
At cryogenic temperatures, thermal transfer is predominantly through direct conduction, since the influences of convection and radiation are negligible. When considering conduction of heat it is necessary to have a good contact between the materials and at the macroscopic level it may seem that two adjoining surfaces are flat, but when looked at on a microscopic level it can be observed that this is not true.
Figure 2 demonstrates how surface roughness can cause the actual surface area in contact between two materials to be extremely small. In fact, due to microscopic roughness the actual contact area can be as low as 1-2% of the total surface area between two materials. ii At room temperature this is less of an issue, since the movement of gas molecules between the two materials helps the heat transfer. However, the process is quite inefficient and after the temperature is close to absolute zero the movement of gas molecules slows and thermal transfer is dominated by conduction.
Figure 2. Surface roughness impact on contact area.
Figure 3 shows the surface of a copper plate at 6000 times magnification, clearly demonstrating the kind of imperfections that can exist, even when the surface appears smooth to the naked eye.
Figure 3. SEM image of a copper surface.
The principle of using a heat transfer grease is simple, the grease offers a direct thermally conductive path between the two materials, as shown in Figure 4, which helps in eliminating all the free space and fills in the peaks and valleys.
Figure 4. Application of a thermal grease.
There are a vast range of heat-sink compounds which are formulated to provide good thermal conductivity and some of these contain metal particles, in order to improve the bulk thermal conductivity. The big disadvantage with these materials for cryogenic use is that the addition of particles increases the space between the contact surfaces and can in fact become detrimental to the system’s thermal transfer characteristics. Two other well acknowledged methods for heat transfer under cryogenic conditions are either Indium foil or specially formulated hydrocarbon grease, which contains additives in order to prevent crazing or cracking at very low temperatures.
When comparing the efficiency of these three solutions, it is possible to look at the complete thermal range from room temperature down to liquid helium temperatures, where low temperature superconductors operate. Figure 5 presents a chart that compares the thermal conductance between copper contacts filled with hydrocarbon grease Apiezon N, a Cu filled grease compound named Cryocon or Indium foil between two copper contacts.ii
Figure 5. Thermal conductance of copper contacts with various thermal interfaces.
It can be observed that at higher temperatures the indium foil gives the best thermal conductance, but as the temperature drops below around 40 K the Apiezon hydrocarbon grease becomes a lot more effective. The cryocon metal filled grease has a higher thermal conductivity when measured in isolation, but in this type of application the advantage of adding metal particles is decreased, as the distance between the contacts is larger in comparison to the other two solutions.
When researching liquid helium temperatures it can be seen that the hydrocarbon grease has much better performance than indium foil when the temperatures drop further to the 1-6 K range, as shown in Figure 6iii
Figure 6. Thermal conductance of copper contacts at liquid helium temperatures.
These experiments highlight that for cryogenic temperatures hydrocarbon grease offers an exceptional thermal contact material, particularly for applications which run in the liquid helium range.
A further benefit to using hydrocarbon grease is the potential to effortlessly remove it. This can be particularly useful when conducting scientific experiments where a sample will have to be temporarily mounted to a cryostat surface. If indium foil is employed, then it becomes extremely complicated to remove the foil once the experiment is complete, whereas hydrocarbon grease can be completely removed by the use of hydrocarbon solvents, such as d-limonene or toluene.
Vapor Pressure and Vacuum Applications
In addition to providing thermal contact, hydrocarbon grease can also provide an excellent sealing medium for vacuum systems that are essential for operating at cryogenic temperatures. In this type of application, key features of the product are the ability to remain crack free down to very low temperatures and to have a low vapor pressure, in order to prevent outgassing of unwanted material into the vacuum environment.
Figure 7 shows the vapor pressure curve for the same Apiezon grease, demonstrating that the product can be used as a sealing compound, as well as a thermal transfer material.
Figure 7. Vapor pressure of grease.
So long as the pressure is maintained above the curve for a given temperature no significant outgassing should happen. For instance, if operating at -10 °C the grease will withstand a vacuum pressure of down to 2.2 x 10-11 Torr, which would be considered high vacuum. As the temperature is decreased into the cryogenic range the vapor pressure will decrease, guaranteeing no outgassing even at ultra-high vacuum.
For cryogenic applications there is frequently a need to provide good thermal conductivity between two surfaces, down to temperatures as low as a few Kelvin. In order to achieve this, a compound is needed which will provide the necessary thermal conductance and survive cycling to extremely low temperatures.
A hydrocarbon grease has been developed and employed for many years in order to provide exceptional performance down to liquid helium temperatures. Besides providing good thermal conductivity, it can be repeatedly cycled back to room temperature, in order to allow the removal and replacement of components, such as test devices on cryostat cold fingers. The grease can also be completely removed with the use of common hydrocarbon-based solvents.
This grease can also be utilized as a vacuum sealant, providing very low vapor pressure, which in turn allows use at very low pressures into the ultra-high vacuum ranges.
i Apiezon N Grease, Cryogenic High Vacuum Grease, Product Datasheet, November 2012
ii Thermal boundary resistance of mechanical contacts between solids at sub-ambient temperatures, E. Gmelin, M. Asen-Palmer, M. Reuther and R. Villar, J. Phys. D: Appl. Phys. 32 (1999) R19-R43
iii Thermal conductance of metallic contacts augmented with Indium foil or Apiezon N grease at liquid helium temperatures, J. Salerno, P.Kittel, A.L. Spivak, Cryogenics 1994, Volume 34, Number 8
This information has been sourced, reviewed and adapted from materials provided by APIEZON.
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